For some characters, class is part of their identity. Both the characters I'm playing now fall under that idea - an Abjurer and a Green Knight, both of whom are basically professional members of their classes.
For others, though, classes are a mechanical means of expressing something about the character, and expressing that something requires multiclassing. They were never defined by the classes written in the books, so it's not "multiclassing" to them when they're multiclass - it's just showing who they are. One of my AL characters, a Rashemi female Lore Bard/Dragon Sorc, is one of these - as far as she was concerned, she was never a Bard, and the idea of Sorcerer as an identity would baffle her utterly. She was a Witch of Rashemen, and no other label would describe her.